Human invisibility is a long-pursued dream that has not yet been completely achieved. Electromagnetic waves refer to radiation whose electric and magnetic field components oscillate in phase, perpendicular to each other, and also perpendicular to the direction of energy propagation of the radiation. The spectrum of electromagnetic waves can comprise all frequencies of electromagnetic radiation including, from low frequency to high frequency, radio, microwave, infrared, visible light, ultraviolet, X-rays and gamma rays. In other words, the spectrum of electromagnetic radiation is infinite and continuous. Generally, incident electromagnetic waves cannot completely penetrate an object they strike; instead, they would at least be partially scattered by the surface of the object. Consequently, a shadow is formed behind the object, creating an area that is not detectable by the incident electromagnetic waves.
An ideal electromagnetic cloaking technology is to guide the incident electromagnetic waves around the object such that the electromagnetic waves would emerge from the other side of the object following their original incident paths without creating behind the object a shadow area where the electromagnetic waves would be blocked. This is as if the object were not irradiated by the electromagnetic waves, or, equivalently, as if the object did not exist, resulting in an ideal cloaking of the object.
One of the current stealth technologies employs painting absorbing materials on surfaces of an object, seeking to minimize reflection of incident electromagnetic waves and thus making the object less detectable. However, this approach does not achieve true invisibility. It works in the microwave frequency range and is effective only for monostatic radars, whereas for bistatic radars or multistatic radars the object can be easily detected.
For visible light frequency range, disruptive patterns are the stealth strategy commonly used; nevertheless, since one pattern cannot be equally effective for all kinds of surroundings, it is just a camouflage rather than a true stealth technology.
In another stealth approach, cameras and displays are employed to mirror the image taken from one side of the object to the other side of the object. The effectiveness of this approach is limited by the quality of the images. Moreover, extra equipment and resources are needed, and the cameras and the associated electrical wires may be visible. Thus this approach does not achieve true cloaking either.
In yet another cloaking approach, optical fibers are utilized as waveguides to direct the incident light from one side of the object to the other side, bypassing the object. This approach requires a large quantity of high quality optical fibers, and can only achieve cloaking in one direction.
In view of their respective limitations, none of the approaches discussed above is considered a candidate for an ideal cloaking technology. As stated earlier, an ideal electromagnetic cloaking technology would guide the incident electromagnetic waves around the object-to-be-hidden such that the electromagnetic waves would emerge from the other side of the object following their incident paths as if they were not blocked by the object, thus creating no shadow area behind the object.
In 2006, J. B. Pendry, Professor at Imperial College London, and his colleagues came up with a method to achieve ideal cloaking. Through a method of coordinate transformation, a cloaking device exhibiting spatially variant permittivity and permeability was designed. Experimental verification of the design was performed in microwave frequencies with metamaterials, composed of inhomogeneous and anisotropic artificial media constructed by metal arrays.
Due to construction complexity of the metal arrays, which would be ever more difficult to implement when applied to a smaller scale, and also due to the intrinsic high loss of metal in the visible light frequency range, the device is practical and effective only in microwave or far infrared frequency ranges. In the experimental verification by Pendry et al., many approximations were made and therefore the device did not achieve the goal of total invisibility, even though it did reduce the scattering cross section area by 24 percent. Nevertheless, the device was still deemed an effective cloaking device because it reduced the shadowed area behind the object to certain degree. In practical engineering, it is hard to remove the shadow area completely because of various non-idealities in the real world.
For all practical purposes, if a cloaking device would cause the shadow behind the irradiated object to reduce, or, equivalently, cause the projected area of the shadow onto a plane perpendicular to the incident wave to decrease, then the cloaking device is considered to be effective. Generally, scientists evaluate the effectiveness of a cloaking device by looking at how much reduction it can achieve in the size of the shadow area behind the irradiated object.
The constitutive parameters obtained through Pendry's method are often inhomogeneous, and include extreme values varying from 0 to infinite, imposing critical criteria to the material used, and also resulting in expensive and difficult realization of the cloaking device. Meanwhile, the material would exhibit a strong dispersion, making the device only workable in a narrow frequency range. Moreover, the device is effective only for electromagnetic waves in certain polarizations, greatly limiting its practical application.